In electronic mail, voice mail, and other information transmission systems that use telephone lines to transmit messages, it is important to test the quality of the telephone line being used. In other words, it is important to test the quality or clarity of the transmissions over the telephone line being used. If there are deficiencies in the transmission of a message because of the quality of the telephone line, there will naturally be undesirable results, including the distinct possibility that the message will not be received in the first place.
The assignee of this invention makes voice mail systems which can transmit verbal messages to specified persons or telephone extensions at remote locations. After a person has recorded a message and has specified the recipients of the message, the voice mail system automatically performs a series of steps to send the stored message to the specified recipients. This may occur immediately after the message has been recorded, or the system may transmit a large number of stored messages at a later time such as at night when telephone usage rates are low.
Referring to FIG. 1, there is shown a block diagram of a message transmission system 20 including a plurality of voice message systems 22, 24, 26 which are interconnected by telephone lines 30. For the purposes of this discussion, the VMS which places a call and initiates a message transmission is called the transmitting VMS, and the VMS which receives the call is called the receiving VMS. Each VMS in the message transmission system is also called a subsystem.
In many circumstances, after the transmitting VMS has transmitted one or more messages to the receiving VMS, the receiving VMS may then transmit one or more stored messages to the transmitting VMS. For this and other reasons, it is important that the telephone line quality be tested for transmissions in both directions between the two VMS's.
Referring to FIG. 2, when the transmitting VMS is ready to transmit one or more stored messages to a receiving VMS, the two subsystems engage in an identification and line testing procedure until a telephone line of acceptable quality is found. The identification procedure, often called a handshake protocol, is performed by exchanging DTMF signals to identify the transmitting system, to confirm the identity of the receiving system, and to identify the destination(s) of the message being sent. The line testing procedure is performed by testing certain aspects of the signals exchanged during the identification procedure. Thus the line testing procedure can be seen as either a second process performed in parallel with the identification procedure, or as a part of the identification procedure.
The feature of the handshake protocol which is relevant to this discussion is that while one VMS is transmitting DTMF signals, the other VMS is not only receiving the DTMF signals for the purpose of decoding the information being sent, it is also testing the DTMF signals received to determine if the line quality of the telephone connection between the two systems is adequate.